1. Field of the Invention
The present invention relates to a method of controlling a switching regulator and in particular to a method of controlling a resonant type switching regulator for providing a d.c. voltage or current.
2. Prior Art
The systems of controlling an output voltage of separately-excited switching regulators include an FM control system in which the turning-on period is constant and the switching frequency (period) is controlled, a PWM control system in which the switching frequency is constant and the turning-off period is controlled and an FM/PWM combined system in which both systems are combined. Each system has its own features. The FM control system has a disadvantage in that the switching frequency varies remarkably and when the switching frequency becomes shifted to a very low frequency range, uncomfortable noise is audible. The PWM control system has a disadvantage in that the switching losses on current interruption increase to lower the conversion efficiency since switching elements are switched on conduction of a resonant current. The FM/PWM combined control system is operated such that the FM control system is operated on a normal operation and the PWM control system is operated only when an output load or an output voltage is lowered.
In contrast to this, a resonant type switching regulator is capable of reducing the switching losses and enhancing the conversion efficiency by performing a switching operation when the current or voltage is zero.
However, a residual voltage is generated across a resonant capacitor in the resonant type switching regulator when the output load becomes low. When the switching regulator is shifted to the next switching operation phase, an excessive resonant current flows due to the residual voltage across the resonant capacitor. Excessive stresses are imposed upon circuit components and switching elements, resulting in high circuit losses.
A resonant type switching regulator shown in FIG. 2 has a resonant circuit including a resonant inductor Lr and a resonant capacitor Cr. If it is assumed that switches S1 and S4 be turned on in initial conditions when the initial voltage V0 across the capacitor voltage Vc and the initial current I0 of the resonant current Ir are zero, a voltage Vc across the capacitor having a wave form shown in FIG. 4(2) is transferred to a d.c. output unit via a main transformer T1.
When the load current-is very low, the current flowing through the smoothing coil L0 in the d.c. output unit exhibits a discontinuous (on/off) characteristic. The resonant circuit functions as follows: The resonant capacitor Cr has been charged so that an initial voltage having a polarity which is determined on completion of the previous operation phase is established thereacross. When the switches S1 and S4 are simultaneously turned on in this condition, the current Ir flows through a path E(+).fwdarw.S1.fwdarw.Lr.fwdarw.Cr.fwdarw.S4.fwdarw.E(-). The current Ir and voltage Vc as shown in the left side of FIG. 5(2) are obtained. When the switches S2 and S3 are turned in the next operation phase, the initial voltage Vc across the resonant capacitor Cr becomes V0.
This relation shows that the voltage V0 is added to the voltage of a d.c. power source E. The circuit current Ir becomes an excessively high resonant current as shown in FIG. 5(2), resulting in large circuit losses.
When the operation periodically proceeds, the primary winding N1 of the main transformer T1 is connected at the opposite terminals thereof to the opposite ends of the resonant capacitor Cr so that a d.c. voltage E0 is obtained across the load resistor R via the secondary windings N2 and N3 and the rectifying circuit including the rectifying diodes D5 and D6, the smoothing coil L0 and the smoothing capacitor C0. If the circuit losses are neglected, the d.c. voltage E0 which-is proportional to the average voltage across the resonant capacitor Cr is obtained.
When a current flows through the smoothing coil L0, an output characteristic is obtained as shown in FIG. 4(2) and a resonant voltage in which the initial value V0 is zero is constantly obtained. When the current flowing through the smoothing coil L0 becomes very low, a discontinuous characteristic of the current is exhibited. An output voltage characteristic represented as Vc in FIG. 5(2) is obtained. The capacitor voltage Vc becomes an excessively higher voltage in comparison with that when a continuous current flows through the smoothing coil L0.